Current efforts at understanding the functioning of the hippocampus in, specifically, spatial information processing, and, in general, memory and learning are weakened by an incomplete knowledge of the role of hippocampal inhibitory interneurons. While a significant diversity of interneuron subtypes has been demonstrated anatomically and electrophysiologically (with the use of in vitro and anesthetized preparations), very little work has been done to identify interneuron subtypes based on unit recordings in behaving animals. Characterization of interneuron subtypes will aid in the understanding of the normal function of the hippocampus and will form a basis for identifying potential disorders of hippocampal interneurons thought to accompany aging and epilepsy. This project was initially designed to generate a basis for interneuron classification in freely behaving animals by combining behavioral paradigms known to modulate interneuron firing properties with pharmacological manipulation of subcortical afferents known to innervate the hippocampal interneuron population. In addition to progress made concerning the main goals of the project, the experiments have generated an important, but unexpected, finding concerning the roles of the dentate gyrus and CA1 hippocampal areas in spatial information processing. Some time has been devoted to handling this data properly. Thus, the present proposal seeks continuation of the original project.